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Natural Selection/Transcript
Transcript Old Version Text reads: The Mysteries of Life with Tim and Moby. Moby is cracking nuts and the only thing left is a peanut. Tim answers a letter. TIM: I can totally see you. MOBY: Beep! TIM: Yes, I can! Update Text reads: The Mysteries of Life with Tim and Moby. Tim is walking around a natural history museum. He stops in front of a display of primate skulls. He stops to examine one skull at a time, when he finally arrives at the head of Moby. Tim screams in shock. Then he notices Moby's headless body standing beside him. Moby's body hands Tim a piece of paper and Tim reads from a typed letter. TIM: Dear Tim and Moby, how does natural selection work? From, Fred D. Hey Fred. There are close to ten million different species of life on Earth today. Natural selection is an explanation for where all of this life came from. An animation shows the screen filling up with different plants and animals from all walks of life. TIM: And what happened to all of the species that are no longer here. Moby reattaches his head to his body. MOBY: Beep. TIM: Sure, but first I gotta tell you a story. Back in the 1800s, a scientist named Charles Darwin went on a historic expedition around the world. A cartoon of Charles Darwin stands in front of a map of the Galapagos Islands. On the map, a small boat traces the course of Darwin's journey from England around the southern tip of South America. TIM: While he was in the Galapagos Islands, he noticed something strange about the birds. Each of the islands had its own unique species of finch. They were all very similar to one another, but their beaks were radically different sizes and shapes. Popups appear over different islands on map of the Galapagos Archipeligo. Each popup bubble shows the image of a finch. Their heads and beaks are different shapes and sizes as Tim describes. TIM: On islands with cactus fruits, the finches longer beaks, which helped them pick out the seeds. On islands with lots of insects, the finches had thin, sharp beaks. And so on. Side-by-side animations show a finch with a long beak picking seeds out of a cactus flower and a finch with a small, thin beak grabbing an ant off the ground. TIM: This got Darwin thinking. What if the finches had a common ancestor, and become more specialized to suit their environments? An animation shows Darwin hiding in the bushes with a pair of binoculars. He lowers the binoculars and begins to think about the different finches he has seen. A family tree of different finches appears above his head. All of the branches connect back to the same ancestor. MOBY: Beep. TIM: Well, it was just a hypothesis, a possible explanation for what he was seeing. It took about 20 years of reading, collecting evidence, and experimentation before he was sure of it. An animation shows Charles Darwin moving through the stages of research Tim describes, and aging over time. As a young man, he sits at his desk poring over a stack of books; then he studies a feather and other specimens with a magnifying glass and microscope; and finally, as an old man, he has a lab full of pigeons he is using in an experiment. TIM: By then, he'd developed an idea for how organisms might change over time. In any population of plants or animals there are variations – random differences. Look at a family of brothers and sisters, and you'll see what I mean. Nobody's exactly the same. The animation shows a close up of a family of pigeons in Darwin's lab. The babies are all slightly different colors with different markings. TIM: Darwin proposed that in nature, certain variations would help organisms survive. MOBY: Beep. TIM: Well, surviving in nature is no easy task. Living things are in constant competition for resources. In the nicest environment, there may be a shortage of food, or water, or even space. Plus, you may get eaten. An animation shows three zebras drinking from a small pool. As Tim describes different environmental threats, the image zooms out to show a fourth zebra trying to get a spot at the drinking pool. Then it zooms out further to show a lion hiding, waiting to attack the zebras. TIM: Those are all environmental pressures: stuff that limits a population of living things! This wasn't a new concept, and neither was variation. Darwin's breakthrough was in seeing how the two ideas came together to create change. An animation shows Darwin in his lab. On a blackboard, he writes: Variation + Pressure = Change. MOBY: Beep. TIM: Okay, imagine a coral reef with some little green fish living near it. There's a big fish that lives there too and he likes to eat the little fish. An animation shows a school of green fish getting attacked and eaten by a larger fish as Tim describes. TIM: As with any population, the green fish have their share of variations. Some differences aren't so good. An animation shows another school of little green fish. All are slightly different shades of green, but one is a deep red. The animation zooms in on the little red fish, which has a scared look on its face. TIM: But others can be seen as favorable. A fish the same color as the coral reef is harder to spot. So it's less likely to get eaten. An animation shows how a little green fish can blend in completely with the coral reef. Overhead, the big fish chases a school of fish with colors that don't blend in. TIM: It has a better chance of living long enough to reproduce. An animation shows the reef-colored fish falling in love with another fish that is a brighter shade of green. TIM: Now, let's say its variation is something that can be passed on to its offspring. Those that get the new color will have a better chance to reproduce. An animation shows a school of offspring. Some of them are bright green and some of them are reef green. The reef green fish are able to hide in the reef, while the bright ones get chased by the big fish. TIM: While the brighter fish succumb to the environmental pressure of being eaten. Over time, the entire population may be the same color as the reef. An animation shows a school of reef green fish camouflaging themselves. TIM: This system is called natural selection. Darwin also referred to it as, survival of the fittest. MOBY: Beep. TIM: The fish that match the reef aren't in better shape than the brighter fish. They're fitter in the sense that they're in a better position to survive. These days, biologists don't really use that term because really, natural selection is driven by the death of the least fit. In our example, that'd be those bright fish that the bigger ones are likely to eat. An animation shows a bright green fish and a reef green fish facing off and flexing their muscles. They are shown hiding in the reef, where the bright fish stands out much more than the reef green fish. The big fish swims by and eats the bright fish, while the reef green fish slowly backs away. TIM: Natural selection is the driving force behind Darwin's theory of evolution. That's the idea that organisms change or evolve over time. An image shows the evolution of humans from an ape-like ancestor to an upright walking human. TIM: There's a huge body of evidence to support evolution, so don't get thrown by that word, theory. Scientists use it in a different way than the rest of us. An animation shows the different skulls Tim was examining at the beginning. There are variations among them, but they all look quite similar. MOBY: Beep. TIM: Evolution works really slowly. It can take millions of years for one species to split off from another. But if you're not convinced that organisms can change just take a look at Sadie, here. An animation shows the silhouette of Sadie who barks. TIM: Sadie's a product of unnatural selection: Humans bred wolves to be more suitable as companions. Some were bred to do hard work; others to be pets. An animation shows a family-tree link between Sadie the dog and a wolf, which howls. Other branches appear, connecting the wolf to the other kinds of dogs Tim describes. TIM: It took about 12,000 years to get from this wolf to this little guy. They aren't separate species just yet… but like I said, evolution is slow. An animation traces a line from the wolf to the silhouette of a Chihuahua. MOBY: Beep. TIM: Live on earth has been evolving for more than three-and-a-half billion years! Plenty of time to produce the amazing organisms we see today. An animation shows a close up of single-celled organisms, and then zooms out to show a whole collection of different animals from all walks of life. TIM: Pick any two species, and go back far enough in time, they'll have a common ancestor, just like Darwin’s finches. An animation shows how a bird and a hippopotamus are connected to a lizard. TIM: In his book, The Origin of Species, Charles Darwin compares the almost endless complexity of life to a massive tree. Some branches don’t work out, and they drop off, becoming extinct. An animation shows a huge tree, with a wide selection scatters throughout its branches. It zooms into a branch holding a dinosaur, which eventually falls off. TIM: Other branches thrive, giving rise to new sub-branches and contributing to the amazing diversity of life. Moby takes off his head and is wearing a skull head Tim screams, then gives a little grimace at Moby. Category:BrainPOP Science Transcripts Category:BrainPOP Transcripts